A strange Cretaceous tree fern

Lately my research assistant and I have been making thin sections of fossil wood from Cretaceous and Cenozoic sites, but last week we made a thin section of a silicified fern. We start by cutting the fossil into a small piece that can fit onto a glass slide. Then, we mount the smooth surface onto the slide with epoxy. Once the epoxy has set we use a grinding wheel to polish the fossil down until it is about 10-20 micrometers thick. At that point light can pass through the fossil and the anatomy of the plant is visible under a microscope.

Tempskya

You are looking at a cross section of a fern stem with three diverging leaf (frond) traces and a surrounding mantle of roots. This is Tempskya sp. Here is a simple sketch of the photo above for comparison. The red are the roots, the and the green represents the vascular tissue in the stem (dark green) and the diverging leaves (light green)

Tempskya sketch

Tempskya was a strange fern unlike any alive today. The small stems and roots collectively formed a thick trunk. Even though the plant produced no wood, it could grow several meters high. In more familiar tree ferns, like those growing in New Zealand, there is only a single stem and the stems gets wider as it grows up, so the base of the trunk is supported by a surrounding mantle of roots. In Tempskya, the stems stay small, and branch continuously forming a tangle that weaves upward through the thick mantle of roots. The leaf traces tend to be produced to one side of the stem and this has led to some rather odd looking reconstructions, but no one has ever discovered the fronds that go with these trunks, so that part is conjecture.

So what kind of fern is it? Some think it is most closely related to the modern ferns of the order Schizaeales, a group that includes some morphologically disparate forms like Anemia, Schizaea, and Lygodium, but it is so different from each of these that it may belong to its own branch of fern diversity near the base of the Leptosporangiate ferns.

ADDENDUM: In the upcoming August issue of the Review of Palaeobotany and Palynology, Martínez and Olivo report foliage with attached sori embedded within a Tempskya trunk from the Cretaceous of South America! Although other plants have been found embedded within the root mantle of Tempskya before, these appear to belong to Tempksya and not an epiphyte or liana. The authors argue that this new information supports a close relationship to Cyatheales, the classic tree fern clade. It seems pretty convincing to me. Perhaps we will see a phylogenetic analysis in the future.

Euanthus pannii

I’ve seen some coverage of this new putative Jurassic flower published online and open-source in Historical Biology a few weeks ago, so I decided I’d share my thoughts. If you didn’t hear about it, here are some links to the open source article and some associated media:

http://www.tandfonline.com/doi/abs/10.1080/08912963.2015.1020423#.VSiGl_nF-Eo

http://www.scmp.com/lifestyle/technology/science-research/article/1747655/worlds-earliest-flower-found-northeast-china

http://www.livescience.com/50419-oldest-flower-fossil-angiosperm.html

First, some background. The oldest angiosperms that can be assigned to living groups are Cretaceous. These are plants that, if you saw them alive, anyone who has taken a botany class would recognize immediately as angiosperms. If we assume that the fossil record as we know it tells a reliable story, then the story of the angiosperm diversification took place during the Cretaceous.

Diversification of Potomac Group angiosperms

Diversification of Potomac Group angiosperms

However, flowering plants are related to other plants, and the other living seed-plant groups to which angiosperms are related (albeit distantly) are conifers, Ginkgo, cycads, and Gnetales. These groups have fossil records that go back much farther than the Cretaceous. This means that the origin of angiosperms is more uncertain. There are extinct seed-plants that lived during the Jurassic and Triassic that are more closely related to angiosperms than to any of the living gymnosperms. These extinct groups would have had some, but not all of those features that unite modern angiosperms and distinguish them from gymnosperms. Some of these extinct angiosperm relatives are certainly still in the rocks waiting to be discovered, but others are probably already in our collections in museums around the world. Most of us paleobotanists have our opinions and hypotheses about which known groups of extinct gymnosperms are angiosperm-relatives; but I wouldn’t say there is consensus yet and there is still so much potential to discover well-preserved highly informative fossils in the field.

The field

The field

As for this recent paper, authors Liu and Wang document what they interpret as an incomplete fossil flower that had 5 sepals, 5 petals, probably 5 stamens, and a gynoecium (pistil) with a unilocular ovary, unitegmic ovules, and a hairy style. Their interpretation of the characters placed the fossil among the living eudicot angiosperms rather than near the base of angiosperm phylogeny, but they did not include a phylogenetic analysis. They also argue that this fossil is mid-Jurassic and therefore one of two things should follow: 1) either modern angiosperms (including groups like monocots, magnoliids, and water lilies) are all many tens millions of years older than we think based on the rest of the fossil record, or 2) the consensus regarding angiosperm phylogeny that has been reached over the last ~20 years of molecular phylogenetics is wrong. While either of these are possible, they are unlikely because most of the evidence of angiosperm evolution fits a different model much better. As well-preserved fossils of modern angiosperm orders and families appear in the Cretaceous fossil record, they do so in a sequence that matches what we expect based on our knowledge of angiosperm relationships, and our knowledge of angiosperm relationships comes from DNA, not from fossils. Furthermore, the semi-independent fossil record of leaves, pollen, and charcoalified flowers all tell the same story of a Cretaceous diversification.

cartoon of angiosperm phylogeny

cartoon of angiosperm phylogeny

As for their interpretation of the morphology of this plant fossil, I have to say I am just not convinced. I see the structures labeled as sepals and petals, but I think they look more likely they are helically arranged rather than whorled like a pentamerous eudicot flower, and while I see some parallel striations, and I don’t see any indication of venation. As for the tetrasporangiate anthers, I don’t see them. The argument appears to be based on the position of these small structures above the petals, and on the constriction in the middle of these small structures. I don’t see the different layers of the microsporangia and the detail view of the pollen grain inside one of the “anthers” doesn’t look like pollen to me. The pollen of eudicot angiosperms is generally both tough and very distinctive. Finally, as for the gynoecium, while I see the thin structure they interpret as a hairy style, I am not convinced by the illustrations of the ovary, or the unitegmic ovules (shouldn’t they be bitegmic in a eudicot angiosperm?), the micropyle (a tiny hole in the integument that the pollen tube grows through), or the papillate inner surface of the ovary. Taken together, their interpretation of all the different organs are consistent, but the uncertainty associated with each part of the fossil makes the final interpretation unreliable in my opinion. It is likely some plant part, but it’s not enough to throw out the last forty years of early angiosperm paleobotany and the last thirty years of molecular phylogenetics, as the headlines may suggest. On the other hand I think Lui and Wang expected that reaction from most of us and they will continue to report on these strange and interesting compression fossils from Northeast China.

Paleobotanical mystery – what is this unidentified plant?

One of the things I love about paleobotany is that any project inevitably leads to new discoveries and new questions. Palebotanists spend a lot of time looking at things that nobody has ever looked at before, so if you like discovery, paleobotany could be for you. As an example, my assistant and I recently found a fossil of a small herb inside of piece of fossil wood from a tree, and now we are trying to figure out what it is and how it got there…

We made this discovery when were examining petrified wood preserved in marine deposits of Panama that are ~18 Million years old. The fact that the wood is preserved in marine deposits means that the pieces were washed out to sea and then fossilized. Sometimes, wood fossils that are preserved in marine deposits have holes made by small wood-boring clams after the wood was washed out, but before it was fossilized. These clams can still be found breaking down wood in the ocean today. Here is a photo of one of our fossils showing the holes that were probably made by wood-boring clams.

Miocene fossil wood

We cut the fossil wood with a rock saw and then examine the cut faces. Normally, we examine the anatomy of the wood, but this specimen turned out to have a tiny plant fossil in one of the holes, seen below cut in cross section. This is strange because if the holes were made by clams, then they were made after the wood was washed out to sea. Unless it was somehow washed in after the clam, the hole must have been made before that to allow this other small plant to grow inside of it.

plant axis in a hole. the words "vascular bundle" are written on the fossil wood tissue, and the the 6-lobed axis is in a hole.

Plant axis in cross section. The words “vascular bundle” are written on the fossil wood tissue that surrounds the 6-lobed herbaceous axis.

We think that this is a stem and not a root because the roots of dicots have vascular tissue in the center, whereas this thing has vascular bundles around the outside, one per lobe. The tissue in the center of the stem was probably thin-walled parenchyma cells that are not preserved. Below is a close up of one lobe with a vascular bundle. The group of cells in the very center of this photo are the water-conducting xylem cells.

close up of a vascular bundle

close up of a vascular bundle

Does anybody recognize what this is? I found an image of a six-lobed stem of Clematis (Ranunculaceae) in cross section HERE that has me intrigued. Whatever it is, I’m sure the fossil record of Neotropical herbs is sparse, so it would be nice to have an identification. Here are two more pictures.

plant axis in a hole in wood.

plant axis in a hole in wood.

plant axis in a hole in wood

plant axis in a hole in wood

Tasmania – where the red fern grew

A few years ago someone gave me a brick-red plant fossil from Tasmania labeled “Osmundacaulis.” Recently, I’ve been making thin sections of petrified wood, but I began by practicing on a few of my own fossils. I started with the Osmundacaulis specimen and in honor of fossil day here are some photos of the fossil in cross section.

Osmundacaulis roots and stipes

What you are seeing is a small part of the outer portion of tree fern trunk, in cross section . The marks across the top are millimeters. You can see a complete trunk in cross section here. All of the preserved tissue is either roots or frond bases (stipes). There are seven stipes in cross-section, but only 5 are mostly complete. An arrow is pointing to one, and there is a close-up of that stipe in the next photo. All of the rest of the reds and yellows are minerals that have either replaced or preserved the tiny roots.

Here is the close up of the stipe at the arrow in the last photo. Notice the oval-shaped band and the U-shaped bundle inside. The outer oval provides support and mechanical strength to the frond, and the U-shaped bundle includes the vascular tissue, the xylem and phloem.

Osmundacaulis stipe

I’ve never been anywhere near Tasmania, but it has a magical place in my mind and I really hope that someday I’ll get to visit.

Also here is my dog.

happy and alive

A fossil fruit from Panama

Follow this LINK to my post on the Panama Canal Project blog

Dracontomelon macdonaldii

Triassic cycadophytes from Arizona

Check out @paleonate‘s Tweet: https://twitter.com/paleonate/status/453896477119496193

This Triassic Cycadophyte is part of a collection I am making this week with three others for the new Smithsonian deep time exhibit!

Triassic

Here is a complete blog post

http://nmnh.typepad.com/smithsonian_fossils/2014/04/triassic-plants.html

 

The University of Maryland article

http://cmns.umd.edu/news-events/features/1582

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